For the effective cancellation of intermodulation distortion generated in nonlinear devices such as mixers, amplifiers, etc., by means of predistortion or post-distortion techniques, it is advantageous to have precise knowledge of the frequency dependence of this distortion. Moreover, it is important to know the nature of the frequency dependence both before and after the nonlinear element. Unfortunately, it is often impossible to physically separate the linear reactive circuitry from the embedded nonlinear element which is, for example, the case in a travelling wave tube. Thus, a direct measurement of the transfer characteristic of the frequency dependent portion of the nonlinear circuit is not feasible. Nor are currently known distortion measuring techniques capable of distinguishing among the various frequency selective effects. For example, U.S. Pat. No. 3,663,954 discloses a method of measuring intermodulation distortion by applying two bands of noise and a fixed tone to the device under test. The measured distortion gives an integrated indication of the level of intermodulation across the entire band of interest. It cannot reveal the nature of the frequency characteristics of the test device. In a second U.S. Pat. No. 4,048,559, the intermodulation produced by a pair of swept frequency signals is measured. The measurement, however, is a function of the reactive circuitry both ahead of and after the nonlinear portion of the test device. As such, there is no way to distinguish the separate effects of the input and the output reactive portions of the device upon the intermodulation. Without this information, the ability to apply predistortion or postdistortion correction to the nonlinear device is seriously impaired.